Get multiselect values on the server side in ASP.NET MVC

You can write a view model that will solve the problem.

public class PurchaseSupplierViewModel
{
    public List<int> SelectedSupplies { get; set; } 
    public List<SelectListItem> Suppliers { get; set; } 
}

and your controller:

public ActionResult YourAction()
{
    List<Supplier> SupplierList = (get all Suppliers from DB here) 
    var model = new PurchaseSupplierViewModel()
    {
       Suppliers = SupplierList.Select(x => new SelectListItem
                        {
                            Value = x.ID.ToString(),
                            Text = x.Name,
                        }).ToList()
    };
    return View(model);
}

[HttpPost]
public ActionResult YourAction(PurchaseSupplierViewModel model)
{
    // model.SelectedSupplies will contain the selected Supplier IDs here
}

then the view:

@model PurchaseSupplierViewModel

@using (Html.BeginForm())
{
    @Html.ListBoxFor(x => x.SelectedSupplies, Model.Suppliers, new { @class = "chosen-select" })
    <input type="submit" value="Submit"/>
}

Here is how I would do it with some test examples:

Model:

    public class SuppliersModel
    {
        public List<Supplier> Suppliers { get; set; }
        public string[] PurchaseQuerySuppliers { get; set; }
    }

    public class Supplier
    {
        public int ID { get; set; }
        public string Name { get; set; }
    }

Actions:

    public ActionResult ListTest()
    {
        var model = new SuppliersModel();
        model.Suppliers = new List<Supplier>();
        model.Suppliers.Add(new Supplier { ID = 1, Name = "Name1"});
        model.Suppliers.Add(new Supplier { ID = 2, Name = "Name2" });
        return View(model);
    }

    [HttpPost]
    public ActionResult ListTest(SuppliersModel model)
    {
        string[] selectedItems = model.PurchaseQuerySuppliers;

        return View(model);
    }

View:

@model SuppliersModel
@using (Html.BeginForm())
{
@Html.ListBoxFor(model => model.PurchaseQuerySuppliers, new MultiSelectList(Model.Suppliers.AsEnumerable(), "ID", "Name"), new { @class = "chosen-select"})
<input type="submit" value="submit" />
}